The present invention relates generally to detecting media defects. More particularly, the present invention relates to a method and apparatus for detecting media defects based on error energy.
Before a disc drive leaves the factory it is scanned to determine the location of media defects that will give rise to problems in reading back data. Many of the errors in magnetic recording systems, such as hard disc drives, are reported to come from sporadic amplitude loss of read back signal. Analysis shows that the main causes of the amplitude loss are media asperity, and lumps or nonmagnetic materials attached to the media surface. All of these causes are referred to as a media defect. The defect may cause variation of spacing between the head and media. As the recording density increases, the head-media spacing will decrease accordingly. A small variation of head-media spacing may cause a significant change in the amplitude of read back signals. Usually a media defect may cause detectors to make decision errors, which may result in errors of several bits to hundreds of bits. Advanced signal processing and error correction code techniques may be used when the media defect is not severe. However, if the media defect is so severe that the stored information cannot be recovered reliably, a safe and straightforward method is to avoid storing information in the area associated with the media defect in the future. This is known as a defect scan and is accomplished by the following two steps:
(i) Identify the media defects.
(ii) Map the locations of the media defects onto a defect table.
The more important of the two steps is to identify the media defects. Many methods have been proposed to carry out this identification.
Currently, one of the most widely used defect scan methods is to write a periodic data pattern onto the media. To detect a media defect during a read back operation, a threshold detector is employed to compare equalized read back signals with a pre-selected threshold. During the media defect scan test, different types of data patterns are selected so that they may have different sensitivity to different types of media defect size. A commonly used detect pattern for media defect scan is a 2T pattern, being a sampled and normalized pattern typically in the form (+1, +1, xe2x88x921, xe2x88x921). Since the data pattern is periodic and fixed, the amplitude of the equalized read back signal should be substantially constant. In other words, the amplitude should only vary within a small range, depending on the noise level. However, if there is a media defect on the disc, the amplitude may drop significantly. When it is less than the pre-selected threshold, it will trigger the threshold detector to locate the media defect locations and map them onto the defect table.
A disadvantage of the current method of defect scan is that there is a possibility that more areas are mapped out than what is required or that those defects that are shallow are missed out. This in turn may reduce both the drive yield and the drive capacity. Some of the xe2x80x98badxe2x80x99 locations could be due to many single-bit errors that are correctable by Error Correction Codes (ECC) and are not true defects. An improved method has been proposed to count the total defect number within a window, which can filter out small media defects causing single bit errors. However, all existing methods are based on signal amplitude, and assume that the read back signal suffers from amplitude modulation. The prior art methods work well at relatively low recording densities. However, as recording densities increase, these methods fail to identify those media defects that are shallow and, which are not obvious at low densities. The prior art methods are very sensitive to channel noise. Therefore, there exists a need for a new defect scan method that can identify shallow defects and that is not as sensitive to channel noise.
The present invention provides a solution to this and other problems, and offers other advantages over the prior art.
The present invention provides a method and apparatus for detecting media defects in a hard disc drive which addresses the above-mentioned problem and is based on error energy.
According to one embodiment of the present invention there is provided a method for detecting defects in a recordable, the method including the steps of computing an error energy and determining if a defect exists based on the error energy.
According to another embodiment of the present invention there is provided an apparatus for detecting defects in a recordable medium, the apparatus including means for computing an error energy and means for determining if a defect exists based on the error energy.
In yet another embodiment of the present invention there is provided a program storage device readable by a computer system tangibly embodying a program of instructions executable by a computer system to perform an error detection method, comprising the steps of computing an error energy and determining if a defect exists based on the error energy. In addition, the invention also can be implemented as a disc drive storage system itself.